Propylene impact copolymers are commonly used in a variety of applications where stiffness and impact resistance are desired such as molded and extruded automobile parts, household appliances, luggage and furniture, etc. Propylene homopolymers are often unsuitable for such applications because they are too brittle and have low impact resistance particularly at lower temperatures, whereas propylene impact copolymers are well suited for applications such as these.
A propylene impact copolymer or heterophasic copolymer or block copolymer generally contains at least two phases or components, a homopolymer component or continuous matrix and a copolymer component or dispersed phase. These two components are usually produced in a sequential polymerization process wherein the homopolymer produced in a first reactor is transferred to a second reactor where copolymer is produced and incorporated within the matrix of the homopolymer component. The copolymer component has “rubbery” characteristics and provides the desired impact resistance, whereas the homopolymer component provides overall stiffness.
A variety of methods and reactor systems have been proposed to prepare such propylene impact copolymers among other types of polymers. For example, U.S. Pat. No. 5,698,642 discloses a process for the gas-phase polymerization of olefins carried out in two interconnected polymerization zones, to which one or more α-olefins CH═CHR are fed in the presence of a catalyst under polymerization conditions and from which the produced polymer is discharged. The growing polymer particles flow through a first polymerization zone under fast fluidization conditions (e.g., fast fluidization section), leave said first zone and enter a second polymerization zone through which they flow in a densified form under the action of gravity (e.g., dense phase section), leave said second zone and are reintroduced into the first polymerization zone, thus establishing a circulation of polymer between the two polymerization zones. Such polymerization schemes are commonly referred to in industry as multi-zone circulating reactors or multi-zone circulating reactor systems. U.S. Pat. No. 5,698,642 also discloses that the multi-zone circulating reactor may be combined with conventional technologies such as in bulk or in the gas phase, either in a fluidized bed or a stirred bed in a sequential multi-stage processes (Col. 9, lines 35-44). See also, e.g., U.S. Pat. No. 6,413,477 and U.S. Patent Application Publication No. 2002/0061264 A 1.
However, past endeavors have yet to provide for new reactor system configurations and polymerization processes that provide for efficient and/or economical methods to produce desired polymers such as impact copolymers.